Intrauterine growth restriction (IUGR) increases the risk for perinatal complications and predisposes for adult disease. However, the pathophysiology underlying IUGR remains poorly understood. Insulin-like growth factor I (IGF-I) is a key regulator of fetal growth and the bioavailability of fetal IGF-I is regulated by insulin-like growth factor binding protein 1 (IGFBP-1), which is primarily secreted by the fetal liver. Phosphorylation of IGFBP-1 leads to ~6-10-fold increase in its affinity for IGF-I, resulting in inhibition of IGF-I function. Our previous studies in HepG2 cells, a well-established model of fetal hepatocytes, show that IGFBP-1 phosphorylation constitutes a highly sensitive mechanism by which IGF-I bioavailability is regulated in response to nutrient and oxygen availability. In addition, we reported that IGFBP-1 phosphorylation is increased at three specific residues in human IUGR fetuses. Given that in IUGR due to placental insufficiency, fetuses are often hypoxemic and have decreased circulating levels of essential amino acids in utero, these findings implicate an important role of IGFBP-1 hyperphosphorylation in the pathophysiology of IUGR. Our recent data in HepG2 cells, demonstrate that IGFBP-1 hyperphosphorylation in response to hypoxia and decreased amino acid levels is mediated by inhibition of mechanistic target of rapamycin (mTOR) signaling and activation of the amino acid response (AAR) signal transduction pathway, which activates protein kinase CK2. However, transformed cell lines may not represent the biology of primary fetal hepatocytes and it is unknown if changes in phosphorylation of fetal IGFBP-1 occurs prior to the occurrence IUGR. We propose to use a well- established baboon model of IUGR to test the central hypothesis that inhibition of fetal liver mTOR signaling and activation of AAR is mechanistically linked to increased IGFBP-1 secretion and IGFBP-1 phosphorylation in primary fetal hepatocytes and that these changes precedes the development of IUGR. Our central hypothesis is supported by compelling preliminary data showing marked inhibition of fetal liver mTOR signaling and increased concentration and phosphorylation of circulating IGFBP-1 many weeks prior to restricted fetal growth become evident in baboons in reponse to maternal nutrient restriction (MNR).
In Aim 1 we will establish the mechanistic link between decreased nutrient availability, inhibition of mTOR, activation of AAR and CK2 and increased IGFBP-1 secretion and phosphorylation in cultured fetal baboon primary hepatocytes.
In Aim 2 we will determine the gestational changes in fetal liver mTOR, AAR and CK2 activity and umbilical blood total IGFBP-1 and IGFBP-1 phosphorylation in response to MNR in the baboon. The proposed work is highly significant because it will improve our mechanistic understanding of the molecular links between reduced nutrient availability and restricted fetal growth and will have a significant and sustained impact because it utilizes a non-human primate model with extensive similarities to human IUGR and therefore highly relevant to the development of restricted fetal growth in women.
Intrauterine growth restriction affects 5-10 % of all babies and increases the risk for injuries at delivery and to develop obesity, diabetes, and cardiovascular disease in childhood and later in life. In this proposal we will explore a novel molecular mechanisms underlying intrauterine growth restriction, involving the phosphorylation of IGF-binding protein-1 in the fetal liver. This work will increase our understanding of how this serious pregnancy complication develops in women and may help identify new diagnostic tools and design novel treatments.